TRANSFORMER

INTRODUCTION

X-ray generate supplies electric power of required amount to X-ray tube.

Works over two separate components

  • Control panel or Console
  • Transformer assembly

Console panel is to control kVp, mA, & exposure time.

Transformer assembly contains

  • High voltage transformer circuit (auto &step up transformer)
  • Filament circuit ( Auto &step down transformer)
  • Rectifier

TRANSFORMER

•A device that either increase or decrease the voltage in a circuit.

•Receives 115V-220V, 60-Hz cycle per second of A.C.

FILAMENT heating requires a potential difference of 10V.

ELECTRON ACCELERATION requires a potential difference of b\w 40,000-1,50,000V

(Transformer to change the potential difference of incoming electric energy to required level.)

CONSTRUCTION

Consists of 2 wire coils

  • PRIMARY COIL
  • SECONDARY COIL

Wound around closed iron core.

Core may be simple rectangle with winding wrapped around  opposite of rectangle.

  • First coil which supplies with electric energy is Primary Coil.
  • Second coil which received modified electric energy is secondary coil .

•The CORE is made up of thin iron sheets of special iron alloy (SILICON ALLOY 3%-6%)

•Each layer is separated with the help of insulating layer (butter paper/ leatheroid paper)

•Layers are clamped very tightly together.

Main purpose of laminating layer is to reduce EDDY CURRENT & HYSTERSIS

PRINCIPLE & WORKING

Work on the principle of ELECTRO MAGNETIC INDUCTION.

Magnetic field is created in the core when current is flows through primary coil

This magnetic field induces current in secondary coil.

Current introduce to primary coil only.  Current in secondary coil can be increase or decrease (based on various factors)

•Current induce in secondary coil is because of polarity change of current induce to primary coil.

•If we connect battery to primary coil and voltmeter to the secondary coil, we will see no fluctuation in voltmeter as magnetic field is in steady state.

•Current flows in secondary coil only when magnetic field is increase or decreases.

•Alternating current is used in transformers because it is produced by potential difference /voltage that change continuously in magnitude and polarity.

•Voltage change with change in flow of current.

•If current flow in one direction, voltage is positive.

•If current flow in opposite direction voltage is negative.

•The major characteristic of  alternating current is that it’s voltage change continuously that produces a continuous change in magnetic field.

•Therefore an alternating current in the primary coil of transformer produce an alternating current in secondary coil.

LAWS OF TRANSFORMER

Laws on which working of transformer is based on are:-

  1. Law of conversion of energy
  2. Faraday’s law of induction

1. LAWS OF CONVERSION OF ENERGY

Transformer never create energy. Increase in voltage is accompanied by decrease in current.

                VP IP=V S IS

VP  =Voltage in primary coil

IP = Current in primary coil

V S  = Voltage in Secondary coil

IS. = Current in Secondary coil

2. FARADAY’S LAW OF INDUCTION

•Voltage in two circuits is directly proportional to the no of turns in two coils.

                           NP/NS.  =   VP/VS

   

NP = No. of turns in primary coil.

NS = No. of turns in secondary coil.

VP = Voltage of Primary coil

VS = Voltage in secondary coil

EFFICIENCY

•Efficiency of transformer is the ratio between output power over input power. •

    Efficiency = output power/Input power

CORE

•A transformer core are always designed so that they form a closed circuit so that magnetic circuit has high high permeability.

•Core is laminated to eliminate eddy current loses.

•Types of core

  1. Core type
  2. Shell type
  3. Cross/H – type

1. CORE TYPE

Primary coil wind on one leg and secondary coil wind on other leg.

Features :-

  • easy to assemble
  • Has a good cooling surface
  • Both windings are made as two halves alternatively.
  • Mostly preferred.

2. SHELL TYPE

  • Primary and secondary coils both are wind up on the central bar.
  • Magnetic circuit is shorter
  • Most efficient design in terms of energy conservation and efficiency.
  • Most commonly used for low voltage power and electronic circuits.

3. CROSS TYPE

  • Modified version of shell type transformer
  • Combination of two shell core at 90° to each other
  • Core surrounded by four legs.
  • Coil is winded over central core
  • Cooled easily
  • Used for larger power transformer

TYPES OF TRANSFORMER

Transformer are classified in 3 types

  1. Step-up
  2. Step-down
  3. Auto transformer

STEP UP TRANSFORMER

•It works on the principle of mutual induction.

•Used for increase the voltage.

•Output Voltage in higher than input voltage.

Construction :-

•Coil is wind around the core bar separately.

•No. of turns in secondary coil is higher.

Working 

  • Voltage transformation depends upon no of turns in primary and secondary coil.
  • Higher the no. of turns in secondary coil, higher will be emf in secondary coil.
  • It increase output voltage.

.Function 

  • Use in high voltage transformer circuit.
  • Conversion of 220V of input voltage into 40,000-1,50,000 V.

Step-down Transformer 

  • Work on the principle of mutual induction as well.
  • Used to decrease voltages.
  • Input voltage is higher than output voltage 
  • Construction:-
    • 2 separate coil wind around core.
    • No. of turns in secondary coil is lesser than primary coil.

Working 

  • Current induce to primary coil, creates magnetic flux.
  • Due to lower turns in secondary coil, flow of voltage would be slower.

Function 

  • Provide voltage to filament circuit of X-ray generator.
  • Convert 220v of input voltage to 10V.

Auto transformer 

  • Works on the principle of self induction.
  • Can increase or decrease voltage as per requirements.
  • Secondary coil turns adjustable.

Construction:-

  • Single winding around around iron core.
  • Primary circuit connect across the coil.
  • Secondary coil have one common point with primary circuit.
  • Other point of secondary coil is adjustable

Working 

  • When current is induced to primary coil, magnetic field flow across the core.
  • Magnetic flux will link with all turns forming the coil, including voltage into each turns of winding.

Example
If 230V are applied between  points A and B connect to  115 turns of the  autotransformer winding the  volts per turn will be 2

Function 

  • Provide voltage for X-rays filament circuit 
  • Provide voltage for primary coil of  high voltage transformer circuit.
  • Convenient location for kVp controller that indicates voltage across X-ray tube.

TRANSFORMER LOSSES

  • Copper loss
  • Eddy Current loss
  • Hysteresis loss
  • Flux leakage .

Eddy current loss

  • When magnetic field change, EMF induced to the core.
  • Due to induced EMF to core, current flow in core.
  • Core start getting heat cause Eddy current loss.
  • Can avoid by using thin sheet of metal insulated with paper.

Copper loss

  • When current flow through resistance, amount of power convert into heat.
  • Copper is used for coil in transformer which is also act as resistance and get heated.
  • By using thicker coil copper loss can be reduced.

Hysteresis loss

  • When current induced to primary coil, iron core gets magnetised.
  • Core magnetised twice in each cycle of a.c.
  • Magnetisation reversed with change in polarity.
  • Due to sudden reversal change of magnetic field, friction between molecules occur.
  • This energy loss due to friction is Hysteresis loss appeared as heat.

Flux leakage

  • All Magnetic flux linked with primary coil is not linked with secondary coil.
  • It is said to be Flux leakage, results in loss of energy.
  • Can be minimised using good core design like shell core.

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4 Comments.

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